Using integrated design to achieve net-zero
Integrated design practices allow a middle school to achieve net-zero energy.
- Know which metrics to use to establish baseline energy numbers for a building.
- Understand the four things needed to design a net-zero energy building (NZEB).
- Gain insight into the interdependency on multiple building systems in an integrated design.
The Hood River Middle School Music and Science Building (HRMS) building was designed by Opsis Architecture, Portland, Ore., with mechanical, electrical, plumbing (MEP), and energy consulting provided by Interface Engineering.
With the emergence of nationally established building performance targets and rising concerns over climate change, the push to design and construct net-zero energy buildings (NZEB) is stronger than ever. To achieve this ambitious goal, teams must eschew traditional design practices and use an integrated design process that seamlessly combines architectural elements, site features, and engineered systems as a cohesively operating whole.
This article aims to summarize some of the key components of the integrated design process and highlight the Hood River Middle School (HRMS) Music and Science Building that was designed using this concept. The building opened in the fall of 2010 and was constructed by Hood River (Ore.) County School District using funding from the May 2008 construction bond. The building houses a music classroom, music practice rooms, a new science lab with an attached greenhouse, and associated offices and support space. The building has been operational now for 3 years and has achieved net-zero energy.
At the start of any project and particularly a net-zero energy project, it is critical to determine baseline energy numbers to establish a benchmark for simulation and design purposes. The most widely referenced database of energy metrics is the Commercial Buildings Energy Consumption Survey (CBECS), which is maintained by the Dept. of Energy. It is a national sample survey that collects energy usage and building characteristics on the stock of commercial buildings in the United States.
Energy consumption is available normalized as an energy use index (EUI) in units of kBtu/sq ft/year, which can be broken down into different end uses (heating, cooling, lighting, etc.). The Target Finder tool provided through the Energy Star website is a great tool for determining a building’s baseline energy rating. For the HRMS building, a baseline EUI of 52 kBtu/sq ft/year was determined with the major energy end uses being heating and lighting.
NZEB needs four things
There are four steps in the integrated design concept that are essential in yielding a NZEB: harnessing natural resources at the site, conservation through building elements, use of renewable energy resources, and measurement and verification (M&V) of intent.
Using site-available natural resources is essential and one of the most cost-effective strategies for achieving net-zero energy. Thus, it should be considered first in the design process. The HRMS project integrates multiple natural resources into the building and systems by using the earth, water, wind, and the sun in various forms to achieve its performance.
Wind drives a natural ventilation system that combines low and high clearstory windows and rooftop ventilators to allow cross and stack ventilation. A red light/green light indicator informs building occupants when outside temperatures are favorable for natural ventilation, which engages students in the management of the building’s energy use.
The earth provides one of the most efficient heat sources for the building’s main HVAC system, a geo-exchange system of tubing that is horizontally looped 10 ft under the school’s adjacent soccer field. The tubing is connected to two internal water-to-water heat pumps that feed radiantly heated and cooled slabs in the building.
An additional energy source for summer cooling comes from a portion of an adjacent stream. The district uses the stream’s snow melt runoff for irrigation during the region’s dry summers, which happens to coincide with the need for cooling during the summer months. Thus, the irrigation water is simply diverted through a heat exchanger when there is cooling demand. This cool water can be circulated through the building’s radiant floor slabs to provide cooling without using a refrigeration process.
Due to the proximity to the Columbia River Gorge, renowned for its windsurfing, it was assumed early on that wind-powered turbines could be an effective renewable energy source. However, an analysis of the micro climate of the site proved wind energy was not as cost-effective as solar. The sun provides renewable energy to the building in several different forms, which will be discussed below.
After analyzing the available site natural resources, the next step is to focus on conservation energy measures within and around the building. This step is where an integrated design process can provide the means to achieve net-zero energy and stay within budget. Combining architectural and engineered elements is the most cost-effective way to produce deep energy savings. Energy conservation measures can be broken into two separate categories: passive and active systems.
Arguably the most important passive system for any NZEB is the envelope. Careful attention was paid to this on HRMS, with R-38 rigid insulation (with lapped layers to avoid gaps) on the roof and R-15 insulation under the radiant slab on grade. The insulated concrete formwork (ICF) walls achieve an overall R-value of 25 and provide excellent thermal mass buffering against Hood River’s seasonal temperature swings. This type of wall uses polystyrene foam interlocking blocks to construct the form work for the walls. Concrete is then poured in stages into the formwork with rebar used for added structural stability. The forms are then left in place and provide the building’s thermal protection layer. Due to their monolithic nature, ICFs can drastically reduce air infiltration and thermal bridging with proper detailing. Last, triple glazing helps reduce heat gain and loss at the windows. The triple-glazed windows are set in wooden frames, which provide a higher level of thermal resistance than more common aluminum frames.
Daylighting is highly recommended for any net-zero energy project to reduce one of the highest energy end uses and is an example of a pseudo-passive strategy. While it takes an integrated control system to dim the electric lights in a space, the sun provides lighting passively through the windows or skylights. To achieve the optimum level of daylight in the HRMS classrooms, the project team performed multiple detailed daylighting studies, balancing the results with those of the energy model. The resulting design combines translucent skylights, clerestory windows, and traditional windows with deciduous vines for seasonal shading allowing views in multiple directions from most building locations. Light-colored acoustic panels help reflect natural light from the clearstory windows deep into the classroom space.